The compound 4-hydroxy-6-methyl-2-pyrone (HMEP) has been used as an intermediate in the production of experimental azo dyes, nematocides, and mercocarbocyanine filter dyes. These uses are disclosed in U.S. Pat. No. 2,034,303, by H. Krzikalla and B. Eistert, assigned to General Aniline Works, Inc., Mar. 17, 1936; U.S. Pat. No. 3,657,426, by P. H. Schroeder, assigned to FMC Corporation, Apr. 18, 1972; and U.S. Pat. No. 3,926,970, by F. Sauter, assigned to Eastman Kodak Company, Dec. 15, 1975, respectively. HMEP has been used as an intermediate for the preparation of 1-(4-chlorophenyl)-1,4-dihydro-6-methyl-4-oxo-3-pyridazine carboxylates and related plant growth regulating agents. These uses are disclosed in German Offenlegungsschrift No. 2,808,795, by T. T. Fujimoto, assigned to Rohm and Haas Company, Sept. 14, 1978; and R. H. Wiley and C. H. Jarboe, J. Amer. Chem. Soc., 78, 624 (1955), respectively.
HMEP has been prepared by (1) decarboxylation of its 3-carboxy derivatives; see E. Suzuki, H. Sekizaki, and Shoji Inoue, Synthesis, 652, 1975; (2) by enzymatic deacetylation of dehydroacetic acid, see K. Abe, S. Nonomura, and C. Tatsumi, Nippon Nogei Kagaku Kaishi, 42, 591 (1968); and (3) deacetylation of dehydroacetic acid (DHAA) in hot 90% sulfuric acid, see W. Borsche and B. K. Blount, Berichte der Deutschen Chemischen Gesellschaft, 65, 827 (1932), J. N. Collie, J. Chem. Soc., 59, 607-609 (1891) and U.S. Pat. No. 3,657,426, supra.
Only the preparation in sulfuric acid appears to have commercial promise. The acidic deacetylation conditions by this procedure are critical since heating DHAA in 85% sulfuric acid produces primarily the rearrangement product 2,6-dimethyl-4-pyrone-3-carboxylic acid; see J. N. Collie and T. P. Hilditch, J. Chem. Soc., 91, 787 (1907). Heating in concentrated hydrochloric acid produces 2,6-dimethyl-4-pyrone hydrochloride; see J. N. Collie, J. Chem. Soc., 59, 619 (1981) and Romanian Pat. No. 56,409, by T. Gostea and A. Maza, assigned to Institutul de Cercetari Chimico-Farmaceutice, Apr. 15, 1974.
The preparation of HMEP from DHAA in sulfuric acid as disclosed by J. N. Collie is described in more detail by Borsche and Blount. The Collie-Borsche-Blount procedure is not suitable for batch production of HMEP because of the enormous rates of heat transfer that would be required. It or any other procedure requiring such large ratios of H.sub.2 SO.sub.4 (3 to 1) and quench water (4 to 1) to DHAA suffers from problems of HMEP losses in the quench mixture, large volumes of acidic waste to be treated or recycled, and low kettle productivity. Moreover, the procedures are run under conditions (ca. two minutes at 130.degree. C. or an unspecified time at 120.degree. C.) that cannot readily be duplicated or controlled on a commercial scale.